Semiconductors having elements from Group II and Group VI of the periodic table, such as mercury cadmium telluride, have been advantageously used in the fabrication of infrared radiation detectors and imagers which operate in the lower infrared frequency band down to the limits of the available long wave length atmospheric window, i.e., at wave lengths of 8-12 microns. The detection of such long wave length radiation, if it is to be done using a detector at only moderate cryogenic temperatures, e.g., at liquid nitrogen rather than liquid helium temperatures, is preferably done using a very narrow bandgap semiconductor, such as Hg.sub.1-x Cd.sub.x Te. Such alloys are referred to generically as "HgCdTe". This pseudo-binary alloy, if it has a composition such as X=0.2, will have a bandgap small enough, e.g., 0.1 V, to be bridged by 12 micron photons. In conventional methods for forming detector arrays, photodiodes have been used as optical detectors and have been interconnected with various forms of image processing circuitry. In the formation of such arrays of photodiodes, vias have been formed within the surface of a HgCdTe substrate, and diodes, each formed at a junction of n-type and p-type semiconductor material, have been associated with each via for forming individual photodetectors, each photodetector forming a pixel within the photodetector array.
In the formation of such detectors, it is important that the vias are precisely formed and that their maximum diameters are limited so that the resulting photodetectors can be closely spaced to enhance the resolution of the resulting photodetector array. Such photodetector structures may be formed of bodies or wafers including compounds of the Group II and Group VI elemental groups of the periodic table, such as HgCdTe, and normally have a zinc sulfide (ZnS) or cadmium telluride (CdTe) layer deposited on the HgCdTe wafer to act as a passivation layer and an insulator for the detector. A photoresist pattern is typically formed above the insulating layer, the photoresist pattern having a plurality of openings mutually spaced in accordance with the desired photodetector array pattern, and in accordance with the resolution desired. Vertically integrated detectors forming such infrared focal plane arrays require that, for each pixel, a via is extended through the HgCdTe wafer.
In the past, various forms of etching and drilling processes have been used to form such vias. In vacancy-doped p-HgCdTe, the vias are generally separate, and spaced from the active (diode) area, and they have been formed using a wet etch process in order to avoid unwanted type conversion. However, the isotropic nature of wet etch processes severely limits the resolution and minimum pixel size obtainable. In order to reduce pixel size, dry etching has been used to form the vias. For example, dry etching techniques utilized for etching HgCdTe and ZnS wafers have, in some applications, used energy from a remote microwave plasma discharge apparatus for activating an etchant to reactively remove selected portions of HgCdTe and ZnS wafers. In other applications, dry etching of a wafer made of Group II and Group VI elements is accomplished by placing the wafer on a substrate in a reaction vessel, evacuating the vessel, and generating an in situ plasma in a gap between two parallel plates within the vessel by applying an electric field across the plates, causing electrons to traverse the gap. The continuous activity of electrons within a hydrocarbon and/or hydrogen gas, such as methane, held at a constant pressure within the reactor causes activation of the gas to disassociated and ionized species, which then etch the exposed areas of the substrate to form the vias. In the past, as exemplified in U.S. Pat. No. 5,157,000, such dry etching processes have been performed on compounds of Group II and Group VI elements, and subsequent procedures such as milling and ion implant processes have been required to form the necessary n/p junctions for forming diodes.
In another process (U.S. Pat. No. 4,411,732), type conversion of a HgCdTe substrate is accomplished by the use of an ion-beam milling process to form photovoltaic infrared detector devices having n/p junctions as the photosensitive junctions. A passivating layer of zinc sulphide is deposited on the upper surface of a material having p-type conductivity characteristics, and at least a portion of the surface of the body is bombarded with ions at very high energy levels, e.g., 500V, to etch away a portion of the body. For example, energetic argon ions or atoms may be directed at the surface at high energy levels. Such high energy ion bombardment disturbs and breaks up the crystal lattice within the substrate, loosening some of the mercury atoms, and thereby produces a sufficient excess concentration of mercury from the etched-away portion of the structure to act as a dopant source for converting an adjacent part of the body into material having n-type conductivity characteristics, thereby forming the n/p junction. Ion bombardment milling processes thus produce a physical, rather than chemical, reaction with the lattice of the substrate, damaging the lattice in order to free the mercury and cause type conversion within the adjacent regions. Such ion milling processes entail several inherent limitations and disadvantages. For example, the high energy bombardment of the Group II-Group VI lattice may not be selective in its reactance with various elements, and although highly anisotropic, redeposition occurs, producing vias of low aspect ratios and thus limiting the pixel size and resolution of the resulting photodiode array. Such ion milling procedures thus entail limitations and disadvantages in that: (1) they are not as flexible as desired for controlling the extent of type converted material, (2) they do not produce vias having sufficiently high aspect ratios for achieving the small pixel size desired for high resolution photodetector arrays, and (3) they are not selective in their milling action against successive layers of various types of materials.